A saved plane of a large Cartesian field costs about 3 s in copies, and a kept native save on Linux is unexplained #346

What happens

Two costs at a saved plane have not been explained.

A large Cartesian plane costs about 3 s, in copies. #59 measured what repeats per saved plane of kerr_beam_384's 1.77 GB field (ledger):

A capillary or radial plane costs 7–25 ms. #59 left this as output-layer work for a pull request of its own. Each copy is also another field-sized allocation at the plane, which matters on runs sized to memory.

A kept native save on Linux. SEP 29 recorded about 4.5 ms for a kept native Adams save, against numba_cpu's 1.6 ms, unexplained, with run totals level (1.398 s against 1.397 s, about 110 saves). The native march (SEP 39) and #59 have changed this path since, and the case it used is no longer in benchmarks/, so re-measure first with a run that saves each step.

Why it matters

A run keeping several planes of a large beam spends seconds per plane outside the physics, and its peak memory sits at the plane, not in the steps.

Context

• _observed_field gives an owning host array. An in-memory store has to own what it keeps, while a file only has to write it, so the copies may differ per handler. The file writer and the in-memory store are where they end.
• A translated problem keeps a full copy of its initial field for the life of a run #5 is the problem's copy of the initial field, estimate_memory does not count the problem's copy of the initial field #6 the estimate that misses it, and The run record is rewritten at every plane: /meta/outcome and Multistep's orders #80 the run record rewritten at every plane. None of them covers this path.
• Why spectrum_to_optics exists: stored spectra have the optics convention, and that must not change.

Read, not run.

To do

• Trace a plane from observation to store on native, for the file and the in-memory handlers, and remove the copies nothing mutates.
• See if the conversion can run in place on a buffer the plane already owns.
• Re-time a kept save on Linux on native, and say what takes the time.
• Check peak memory at a plane against estimate_memory.

Stored fields stay bitwise equal, and a kept plane must not alias the live state.

Done when

On 2x Xeon Gold 6240 a saved plane of kerr_beam_384 costs at most half of the 3 s above, timed the same way, with bitwise-equal output and a test that a kept plane does not change when the run goes on. The kept save's time on the same machine is recorded, with what takes it.

Checked against commit 8a9f764. Takes over the roadmap's entry "The cost of a kept native save on Linux."